1.Technical Field
The present invention relates to adhesive compositions for use in bonding fiber reinforced engineering thermoset or thermoplastic materials. More particularly, it relates to adhesive compositions containing electrically conductive materials such as carbon black in combination with ferromagnetic materials to provide a synergistic effect to improve the processing of induction accelerated adhesives especially when low frequency induction coils are used.
2. Background Art
Manufacturers of products that use fiber reinforced engineering thermoset or thermoplastic materials for structures rely on adhesive bonding to join these materials. Typically, in the case of automotive applications a class A paintable surface grade of sheet molding compound (SMC) fiber glass material is bonded to an inner reinforcing member. When manufacturing parts at a rate of one per minute or less, a fast bonding process is required. Traditionally, a two component adhesive was applied, the parts mated, and then held in contact over and under by electrically or steam heated tooling to accelerate the adhesive to a gelled state by thermal conduction. Once gelled, the assembly is dimensionally stable and can be moved off line. The adhesive will then cure to full strength down line at ambient temperatures. Heated fixture tooling must first heat the SMC and then conduct heat into the adhesive to cause the gelation. Two and a half to three minutes was a typical bonding cycle. Advances in SMC molding technology have reduced part molding times below one minute; consequently, short bonding cycles are required to keep pace.
Induction heating has been employed to speed up the bonding process. The adhesive is modified by suspending ferromagnetic particles in the polymer. When placed over a high frequency (450 kHz or higher) current, the induced magnetic field causes the ferromagnetic particles to heat up and dissipate their heat to the adhesive thereby gelling the polymer matrix in about 40 seconds. Induction heating eliminated the need for two side access heated fixtures since the adhesive could be heated directly with one side access induction tooling.
This process of inductively heating adhesives containing only ferromagnetic particles, however, requires a high frequency current (450 kHz or higher) to create a magnetic field that could couple to the small particles in the adhesive. These high frequency generators are based on vacuum tube technology. They are costly and inefficient in operation. High power losses are suffered when the high frequency current is transmitted to the induction coil by a solid copper bar. In order to cope with the transmission line losses, the transmission line must be short in length with high frequency induction generators. This places the generator and the coil in close proximity causing congestion in the immediate work area. High operating voltages (5000 - 8000 volts) are used with high frequency induction generators. Because of the high operating voltages, arcing is a major concern when designing high frequency induction coils and associated work stations. Arcing is a severe safety hazard requiring many safeguards to prevent electrical shock. High transmission line losses also require that the induction coil be fitted with expensive magnetic concentrators in order to maximize the field's effectiveness. The inefficiencies also result in the use of high volumes of cooling water. Because the high frequency induction generators are based on vacuum tube technology, the maintenance of these machines is high. High frequency induction generators may interfere with radio transmission in the local area as well as other electronic equipment in the immediate area. As a result, all 450 kHz generators must be FCC certified.
Accordingly, it is an object of the present invention to provide improved adhesive compositions which allow SMC and other plastics to be bonded in less then one minute with improved processing by virtue of their ability to be heated by low frequency induction generators.
Another object of the present invention is to provide novel adhesive compositions useful in induction heat bonding substantially free of the drawbacks currently known for high frequency induction heating in terms of complexities in design and implementation.
The present invention is also capable of being employed to advantage with high frequency generators. Used in this manner even shorter cycles are possible.
These and other objects and features of the invention as well as the advantages thereof can be fully understood by reference to the following description and claims.